Axis 4 : DYNAMIC PROCESS

The "Probing Dynamic Processes" research axis focuses on dynamic processes and real-time control of surface phenomena upon the interaction with a processing tool, either light-driven or mechanical.

Two particular directions will be explored : dynamics of ultrafast laser-induced processes on materials ; dynamics of transient phenomena occurring at contact interfaces.

The research thrust is to determine the characteristic time and space scales associated with transient processes that occur during material processing. In the course of the interaction between two surfaces or upon laser excitation, a confined medium with small space dimensions is formed (typically a nm to µm). Within this confined interface, fast dynamic phenomena occur at different characteristic timescales, from ps to ms, mirroring a range of non-equilibrium, thermodynamic and thermomechanical processes. Knowledge of relaxation dynamics is essential for developing real time monitoring techniques and intelligent process control approaches. This research and development axis will also produce advanced tools and methods for observing, measuring, and controlling transient phenomena. It will also advance our understanding of processes by developing predictive theoretical or numerical multi-scale models to simulate interfacial phenomena. The emphasis is on local phenomena in confined environments with the analysis of the collective dynamics during excitation or interactions at interfaces and their consequences.

Research directions :

Transient (fast) phenomena are frequently observed under tribological stress but their influence on the evolution and degradation of a tribo-system is still largely unknown. However, advances in modeling and new acquisition techniques now make it possible to understand non-stationary regimes. These phenomena are related to the dynamics of the mechanical conditions but also to the dynamic response of the interfacial material resulting from the contact interaction. Indeed, a confined medium is formed during a dry or lubricated contact. The medium consists in an interfacial layer or a lubricant film whose tribological behavior and associated dynamics depend on its intrinsic properties (nature, mechanical properties) and on the effects of confinement. The latter result in :

• A structuring effect near the walls or, on the contrary, local disorder,
• A geometric confinement in which the thickness of the lubricant film is small compared to the size of the contact, leading to the amplification of perceived macroscopic mechanical stress (high stress gradient or localized overstress, shear rates up to 106 s-1). The interface can then exhibit complex nonlinear behavior (alignment of molecules, dynamic transition between solid- and liquid-like states) or instabilities (localized deformation, cavitation…).

Two of the phenomena for lubricated contact are illustrated below.

Dynamics of confined lubricated interfaces – Contact interferograms (IRIS tribometer, LTDS).

Another example, in dry conditions, is related to the process of friction stir welding. A textured pin rubs against a solid surface at high speed. This contact interaction generates local heating and stress gradients inducing significant microstructural changes at the interface. These structural changes depend not only on the local stress conditions but also on the materials used and their surface topography, as they regulate the friction response of the interface, thus controlling the quality of the process and its implementation. This process raises a series of fundamental questions concerning the transient local conditions, their access and controllability: How can these local phenomena, localized both in time and space, be precisely characterized in the black box that represents the interface.

A major emerging scientific topic of this axis is the identification of local phenomena occurring in a confined area.

The interaction regime and the associated process accuracy in ultrafast laser structuring of material is often associated with the degree of laser-induced nonequilibrium. This makes it possible to explore novel structural phase transitions and metastable states capable of generating new classes of properties and material forms with updated functionalities.
Precise knowledge of excitation channels and relaxation dynamics is a major challenge in material processing techniques today, but at the same time provides guidelines for optimizing laser phenomena. Several tasks will be undertaken :

• Probing ultrafast laser phenomena at subwavelength scales : this task involves evaluating excitation transients, collective excitation dynamics, and primary hydrodynamic steps in laser nano- and micro-processing associated with a change in topology at nanoscale.
• Spatio-temporal pulse manipulation techniques and process control/optimization : this task will produce optimization methods based on the automated pulse design in space and time with the main focus on optimizing laser-matter interactions and increasing their effectiveness.
• Simulation tools to predict interface phenomena : understanding laser-induced processes is indispensable for overall control of the process. This task involves the theoretical and numerical investigation of electronic excitation and subsequent thermodynamic material trajectories as a way of mastering laser processing events.

Conceptual design for probing and controlling laser-induced phenomena and structuring processes.